The Wadden Sea is one of the world's seas whose coastline has been most modified by humans,[3] via systems of dikes and causeways on the mainland and low-lying coastal islands. The Wadden Sea stretches from Den Helder in the Netherlands in the northwest, past the great river estuaries of Germany to its northern boundary at Skallingen north of Esbjerg in Denmark along a total length of some 500 km and a total area of about 10,000 km2. Within the Netherlands it is bounded from the IJsselmeer by the Afsluitdijk.

The islands in the Wadden Sea are called the Wadden Sea Islands or Frisian Islands, named after the Frisians. These are remnants of the once expansive and now submerged Doggerland. However, on the westernmost Dutch island, Texel, the Frisian language has not been spoken for centuries. The Danish Wadden Sea Islands have never been inhabited by Frisians. The outlying German island of Helgoland, although ethnically one of the Frisian Islands, is not situated in the Wadden Sea.

The word wad is Dutch for "mud flat" (Low German and German: Watt, Danish: Vade). The area is typified by extensive tidal mud flats, deeper tidal trenches (tidal creeks) and the islands that are contained within this, a region continually contested by land and sea. The landscape has been formed for a great part by storm tides in the 10th to 14th centuries, overflowing and carrying away former peat land behind the coastal dunes. The present islands are a remnant of the former coastal dunes.

The islands are marked by dunes and wide, sandy beaches towards the North Sea and a low, tidal coast towards the Wadden Sea. The impact of waves and currents, carrying away sediments, is slowly changing the layout of the islands. For example, the islands of Vlieland and Ameland have moved eastwards through the centuries, having lost land on one side and grown on the other.

The Wadden Sea is famous for its rich flora and fauna, especially birds. Hundreds of thousands of waders (shorebirds), ducks, and geese use the area as a migration stopover or wintering site, and it is also a rich habitat for gulls and terns.[4] However, the biodiversity of Wadden Sea today is only a fraction of what was seen before exploitation by humans; for birds, larger species such as geese,[5]eagles, flamingos, pelicans, and herons used to be common as well.[6] Some species that are regionally extinct are still available here.[7][8]

Wadden Sea is an important habitat for two species of seals, harbor and grey seals. Harbour porpoises and Atlantic white-beaked dolphins (seasonally) which once were locally extinct but have re-colonized into the area, and these two are the only resident cetaceans in present days[11] while many other species have either disappeared or only visit seasonally or occasionally.[12][13] Nowadays, only 4 species of marine mammals above-mentioned could be counted as regular inhabitants of Wadden sea while many other species that experienced severe declines, habitat loss, and local or functional extinctions.[14]North Atlantic right whales and gray whales[15] were once seen in the region, using the shallow, calm waters for either feeding and breeding before they were completely wiped out by shore-based whaling.[16] These two species are now thought to be either extinct or remnant populations of which low-tens at best survive. One whale, possibly a right whale, was observed close to beaches on Texel in the West Frisian Islands and off Steenbanken, Schouwen-Duiveland in July 2005.[17] Recent increases in number of North Atlantic humpback whales and minke whales might have resulted in more visits and possible re-colonization by the species to the areas especially around Marsdiep.[18][19] Future recovery of once-extinct local bottlenose dolphins is also expected.[11]

Although the Wadden Sea is not yet listed as a transboundary Ramsar site, a great part of the Wadden Sea is protected in cooperation of all three countries. The governments of the Netherlands, Denmark and Germany have been working together since 1978 on the protection and conservation of the Wadden Sea. Co-operation covers management, monitoring and research, as well as political matters. Furthermore, in 1982, a Joint Declaration on the Protection of the Wadden Sea was agreed upon to co-ordinate activities and measures for the protection of the Wadden Sea. In 1997, a Trilateral Wadden Sea Plan was adopted.[21]

In June 2009, the Wadden Sea (comprising the Dutch Wadden Sea Conservation Area and the German Wadden Sea National Parks of Lower Saxony and Schleswig-Holstein) was placed on the World Heritage list by UNESCO.[23] The Danish part was added to the site in 2014.